Effect Of Shot Peening On Hydrogen Diffusion And Embrittlement In Low Alloy Steel

Since modern times,the increasing energy demand of human beings has led to the over-exploitation and consumption of non-renewable fossil energy.This consequence and the environmental problems brought by it make human beings face energy crisis.Many developed countries around the world have attached attentions to hydrogen energy due to its advantages such as clean,efficient and abundant.China also pays attention to the development of hydrogen energy.At present,domestic hydrogen energy projects have entered the practical use stage.Materials(especially metals)are prone to hydrogen embrittlement(HE)(hydrogen degrades the plasticity and ductility of many metals)because of its particular properties.The uses of hydrogen energy further elevate the requirement for hydrogen compatibility of related material for components such as hydrogen storage vessels and transport pipes.Low alloy steels are the most widely used steels for pressure vessels and pipes.Shot peening(SP)is a kind of surface strengthening method with proven technologies and low cost for low alloy steels,which can improve the fatigue,corrosion and wear resistance of steels.It is necessary to study and understand the effect of SP of HE.In this thesis,the changes of HE resistance of low-alloy steels after SP were studied,and the research methods include finite element simulation,experimental verification and theoretical analysis.The main research contents and conclusions are as follows:(1)A previous study has reported the effect of SP on HE of PSB1080 steel.To explain the experimental observations,finite element(FE)simulations of hydrogen diffusion in PSB1080 steel after SP were carried out by ABAQUS program.Firstly,SP process was simulated to obtain the residual stress and plastic strain fields based on John-Cook model.Then,the obtained strain and stress fields were defined as initial conditions,and Oriani-Sofronis diffusion model,which considers the combined effect of stress and plastic deformation on diffusion,was used to calculate the distributions of hydrogen concentration in the normal lattice interstitial sites and the changes of apparent diffusion coefficient under different conditions.The results indicate: 1)Compared with residual stress,the role of plastic deformation in hydrogen diffusion is more important;2)In a mild hydrogen containing environment,the increase of dislocation trap sites due to SP plastic deformation reduces significantly the apparent hydrogen diffusivity by trapping a large amount of hydrogen,and additionally reduces the resulting hydrogen concentration level in the normal lattice sites and in the trap sites of grain boundaries,inclusions and voids,thus suppressing the HE susceptibility of the PSB1080 steel;3)In a severe hydrogen containing environment,SP has little influence on apparent hydrogen diffusivity.Since a large amount of hydrogen transports into the steel and is trapped at inclusions and voids,leading to hydrogen blisters,HE after SP is enhanced.(2)The surface properties and hydrogen diffusion coefficient of Q345 R steel after SP at different times were studied.Through the comparative analysis of the changes of surface properties and the changes of hydrogen permeation curves,it was found that the hydrogen diffusion coefficient showed a step-down trend with the increase of SP time: 0.5 min SP reduced the hydrogen diffusion coefficient,whereas2~35 min SP has little influence on the coefficient.When the SP time was increased to60,90 min,the coefficient was reduced significantly again.The residual compressive stress and the increase of dislocations induced by short time SP can explain the reduction of the hydrogen diffusion coefficient.However,since on one hand the residual stress has limited influence on diffusion as compared with plastic strain,and on the other hand the dislocation density become saturated after a certain SP time,2~35 min SP has little influence on the coefficient.60,90 min SP leads to the refinement of pearlite and ferrite grains significantly,as a result the interfacial area of pearlite and ferrite grains increases significantly.As the interfaces of pearlite and ferrite grains are strong hydrogen trap sites,90 min SP further reduces the diffusion coefficient.(3)Based on electrochemical cathodically hydrogen charging,room temperature slow strain rate tensile tests and fracture morphology analysis,the influence of SP time increase on HE resistance of Q345 R steel was studied.It was found that with the increase of SP time,the HE susceptibility of Q345 R exhibited a decreasing trend on the whole,but 2~15 min SP had little effect on the HE susceptibility.With the increase of SP time,the center regions of the hydrogen-charged samples gradually changed from mixed quasi-cleavage(QC)and micro-void coalescence fracture(MVC)to only MVC,while the edges of the samples gradually changed from MCV to mixed QC and MVC.The results indicate that since the long-term SP can significantly reduce the hydrogen diffusion coefficient due to the increase of dislocations and the formation of grain refinement,the depth of hydrogen transport into the samples is reduced significantly.Hydrogen is mainly trapped in the surface layer of sample,consequently the hydrogen concentration in the center of sample is reduced,reducing the HE susceptibility of the sample.